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Supramolecular gels are generally formed by the self-assembly of organic molecules called low-molecular-weight gelators. However, supramolecular gels can also be formed with the assistance of metal ions. These organic-inorganic hybrids are key for creating highly functional materials. Unique properties and functions that cannot be obtained from organic molecules alone are observed in the metal ion containing supramolecular gels. In this study, several examples of supramolecular gels formed with the assistance of metal ions, cholesterol derivatives, amino acids and peptides, nucleic acid derivatives, oxalic acid, and C3-symmetric tris-urea derivatives are introduced. This provides an overview of existing supramolecular gels to better understand the scope for possible future research for their practical applications. [ABSTRACT FROM AUTHOR]

Maltose is a ubiquitous disaccharide produced by the hydrolysis of starch. Amphiphilic ureas bearing hydrophilic maltose moiety were synthesized via the following three steps: I) construction of urea derivatives by the condensation of 4‐nitrophenyl isocyanate and alkylamines, II) reduction of the nitro group by hydrogenation, and III) an aminoglycosylation reaction of the amino group and the unprotected maltose. These amphiphilic ureas functioned as low molecular weight hydrogelators, and the mixtures of the amphipathic ureas and water formed supramolecular hydrogels. The gelation ability largely depended on the chain length of the alkyl group of the amphiphilic urea; amphipathic urea having a decyl group had the highest gelation ability (minimum gelation concentration=0.4 mM). The physical properties of the supramolecular hydrogels were evaluated by measuring their thermal stability and dynamic viscoelasticity. These supramolecular hydrogels underwent gel‐to‐sol phase transition upon the addition of α‐glucosidase as a result of the α‐glucosidase‐catalyzed hydrolysis of the maltose moiety of the amphipathic urea. [ABSTRACT FROM AUTHOR]

Urea derivatives that were substituted with a 2‐benzylphenyl group and an alkyl group functioned as low molecular weight gelators for various organic solvents and ionic liquids. Urea derivatives with long alkyl chains were effective for the gelation of polar solvents. However, they were not suitable for the gelation of non‐polar solvents, whereas urea derivatives with short alkyl chains were effective. Ionic liquids were similar to polar solvents in that urea derivatives with long alkyl chains were the most effective gelators. The physical properties of the formed supramolecular gels were analyzed by dynamic viscoelasticity measurements using a rheometer. [ABSTRACT FROM AUTHOR]

Urea, which has both hydrogen bond acceptor and donor moieties, is an ideal structure for a supramolecular synthon. Various supramolecules having ureido group(s) have been widely developed. This article summarizes recent developments of urea derivatives that exhibit various functions: i) supramolecular capsules that form discrete urea–urea intermolecular hydrogen bonds, ii) supramolecular polymers that form continuous urea–urea intermolecular hydrogen bonds, iii) supramolecular gels that form continuous urea–urea intermolecular hydrogen bonds, iv) artificial host molecules based on the molecular recognition ability of the ureido group, and v) catalytic reactions developed by utilizing the molecular recognition ability of the ureido group. [ABSTRACT FROM AUTHOR]

Abstract: The successful development of stiff supramolecular gels is an important goal toward their practical application. One approach to stiffen supramolecular gels is to introduce covalent cross‐links. The bis‐urea dimer 2, having a structure similar to that of the low‐molecular‐weight gelator 1, was synthesized. Supramolecular hydrogels were formed from mixtures of 1 and 2 in appropriate ratios, with 2 acting as a covalent cross‐linker to connect the fibrous aggregates formed by the self‐assembly of 1. The introduction of these covalent cross‐links greatly influenced the dynamic viscoelasticity of the supramolecular hydrogels. In the supramolecular hydrogel of 1 mixed with 5 % 2, the storage modulus was 1.35 times higher than that of the supramolecular hydrogel of 1 alone, and the crossover strain was extended from 5 % to over 20 %. The supramolecular hydrogel of 1 and 2 was free‐standing and supported 13 times its own weight. [ABSTRACT FROM AUTHOR]

The development of an effective adsorbent for cleansing polluted water is required for environmental purification. In this respect, a supramolecular hydrogel constructed by the self-assembly of small molecules could be a strong candidate. Adsorption experiments of organic dyes were performed using supramolecular hydrogels of amphiphilic tris-urea 1. Cationic organic dyes were adsorbed efficiently; indeed, the adsorption of methylene blue was as high as 4.19 mol equivalents relative to 1. Two luminescence peaks were observed in the rhodamine 6G-adsorbed supramolecular hydrogels, and their ratios varied with the amount of dye adsorbed. Fluorescence microscopy images of the supramolecular hydrogel at lower dye levels exhibited fibrous fluorescence consistent with the fibrous aggregates of 1. According to these results, adsorption may proceed gradually, that is, occurring initially on the fibers and later in the aqueous spaces of the supramolecular hydrogel. [ABSTRACT FROM AUTHOR]

This article describes recent developments in C3-symmetric tris-urea low-molecular-weight gelators and their applications. The C3-symmetric tris-ureas are excellent frameworks to form supramolecular polymers through noncovalent interactions. In organic solvents, hydrophobic tris-ureas form supramolecular gels. Amphiphilic tris-ureas form supramolecular gels in aqueous media. Functional supramolecular gels were prepared by introducing appropriate functional groups into the outer sphere of tris-ureas. Supramolecular hydrogels obtained from amphiphilic tris-ureas were used in the electrophoresis of proteins. These electrophoreses results showed several unique characteristics compared to typical electrophoreses results obtained using polyacrylamide matrices. [ABSTRACT FROM AUTHOR]

Two molecules of cavitand tetraboronic acid and four molecules of various bis(catechol) linkers self-assemble into capsules through the formation of eight dynamic boronic ester bonds. Each capsule has a different cavity size depending on the linker used, and shows particular guest encapsulation selectivity. A chiral capsule made up of the cavitand and a chiral bis(catechol) linker was also constructed. This capsule induces supramolecular chirality with respect to a prochiral biphenyl guest by diastereomeric encapsulation through the asymmetric suppression of rotation around the axis of the prochiral biphenyl moiety. [ABSTRACT FROM AUTHOR]

Hierarchical self-assembly of an amphiphilic tris-urea in aqueous media is shown. A mixture of the amphiphilic tris-urea and an alkaline solution gave a viscous solution composed of fibrous aggregates. This viscous solution transformed into supramolecular hydrogels, which are capable of hierarchically organizing into higher-order aggregates in response to several cationic triggers. The resulting supramolecular hydrogels were relatively stiff and their storage moduli attained over 103 Pa. The stimuli-responsive and optical properties of the resulting hydrogels were influenced by the cationic trigger. Proton and calcium ion triggers gave pH- and chemical stimuli-responsive hydrogels, respectively. A terbium ion trigger also provided a highly luminescent hydrogel through energy transfer from the tris-urea to terbium. [ABSTRACT FROM AUTHOR]

A C3-Symmetric tris-urea derivative self-assembles into either a discrete dimer or a supramolecular gel via continuous assembly, depending on the solvent used. In the less polar CDCl3, the discrete dimer was selectively formed, whereas in the more polar acetonitrile and acetone, a supramolecular gel was obtained. Additionally, the gel-sol phase transition of an acetonitrile gel of the tris-urea derivative responds bidirectionally to chemical stimuli. [ABSTRACT FROM AUTHOR]

A C2-symmetrical tetrakis(arylethynyl) cavitand was synthesized. Spontaneous formation of hybrid hydrogen bonded/metal-ligand coordination supramolecular capsules occurs upon mixing 1 with cis-coordinated Pt2+ or Pd2+ metal complexes. The hybrid supramolecular capsules encapsulated neutral guest molecules only in polar solvents. [ABSTRACT FROM AUTHOR]

Hybrid supramolecular capsules self-assemble by simultaneously forming hydrogen and metal-ligand coordination bonds on mixing a C2-symmetrical cavitand (calix[4]resorcinarene-based cavitands with ureide and terminal 4-pyridyl units) with platinum or palladium complexes ([Pt(OTf)2] or [Pd(OTf)2] with chelating bisphosphines) in 1:1 ratio. Hemicapsular assemblies formed in the presence of excess amounts of cavitand relative to the platinum or palladium complexes are identified as intermediates in the above self-assembly process by 2D-NOESY spectroscopy. External-anion-assisted encapsulation of a neutral guest, 4,4′-diiodobiphenyl, inside the hybrid supramolecular capsules accompanied conformational changes in the hydrogen-bonding moieties. The in/out exchange ratio of the encapsulated guest depends on the bite angle of the bisphosphine ligand. Addition of DMSO accelerates guest exchange by weakening the hydrogen bonds in the encapsulation complex. Therefore, variations in the structure of the metal complex and amount of polar solvent exert dual control on the dynamics of the guest exchange. [ABSTRACT FROM AUTHOR]

Transition stress management: The amphiphilic tris ‐ urea 1 was designed and synthesized as a low ‐ molecular ‐ weight gelator. The minimum gelation concentration of 1 is 1.9   mM. The hydrogel is thixotropic, and shows a reversible gel–sol phase transition in response to mechanostress. The amphiphilic tris ‐ urea 1 forms gels with various aqueous solutions, such as buffers and saturated NaCl, 8 M HCl, and 7 M KOH solutions. These hydrogels retain their thixotropic nature. [ABSTRACT FROM AUTHOR]

A mixture of tris-urea 1 and acetone formed a gel on ultrasound irradiation. The gel-to-sol phase transition proceeded by adding halide ions as tetrabutylammonium (TBA) salts. The sols (solutions) were able to regel by mixing them with other metal salts followed by ultrasound irradiation. The regelation ability of various metal salts was tested. Anion selective and non-selective regelation was observed depending on the metal salt used. The mechanistic aspects of the sol-to-gel phase transition in the presence of metal salts were investigated using 1H NMR employing a model compound. [ABSTRACT FROM AUTHOR]

Abstract: Electrophoresis of denatured proteins was performed in a supramolecular hydrogel matrix constructed by the self‐assembly of amphiphilic tris‐urea 1 in the tris‐glycine‐sodium dodecyl sulfate (SDS) buffer. Proteins were separated based on two different molecular sieve effects in this electrophoretic method. Smaller proteins showed a larger mobility than larger proteins as in SDS‐PAGE with various large proteins. In contrast, smaller proteins showed a poorer mobility than larger proteins, similarly to gel filtration conducted for various small proteins. The dominant separation mode changes at a certain molecular weight of the protein and this threshold is influenced by the concentration of SDS. [ABSTRACT FROM AUTHOR]

Abstract: C 3-Symmetry tris-urea low molecular weight gelator (LMWG) (1), which shows chemical stimuli responsible for a sol–gel phase transition, was divided into five regions. Based on the division, 22 derivatives were synthesized. The gelation ability of these derivatives was tested in nine organic solvents with a wide range of values for relative static permittivity (ɛ r=47.2–1.89). Some derivatives showed a better performance as LMWGs than the original tris-urea LMWG (1). For example, the critical gelation concentration (CGC) in acetone was improved from 1.5wt % to 0.5wt % by changing the core substituent (18). Highly versatile LMWG for a variety of solvents was obtained by changing the linker moiety (23). Structural information to design tris-urea LMWGs is important to create rationally a functional supramolecular gel. [Copyright &y& Elsevier]

Abstract: Tris–urea 1 functioned as a low-molecular-weight gelator for a variety of polar organic solvents. An acetone gel of 1 became a homogeneous solution in response to the addition of anions. The minimum amount of anion necessary for the gel–sol transition was specific to the anion species. A linear relationship was demonstrated between the amount of anion required and the total binding constant of 1 with the anion. Re-gelation occurred by addition of BF3·OEt2 and with ultrasound irradiation of the acetone solution of 1 and F−. [Copyright &y& Elsevier]

Ionic liquids are similar to polar solvents in that urea derivatives with long alkyl chains are the most effective gelators. Keywords: gels; alkyl group; self-assembly; rheology; urea EN gels alkyl group self-assembly rheology urea 1643 1643 1 07/05/21 20210705 NES 210705 B Urea derivatives b that were substituted with a 2-benzylphenyl group and an alkyl group function as low-molecular-weight gelators for various organic solvents and ionic liquids. However, they are not suitable for the gelation of non-polar solvents, whereas urea derivatives with short alkyl chains are effective. [Extracted from the article]

In their review on page 5601, M. Yamanaka et al., summarize recent developments of urea derivatives that exhibit various functions: i) supramolecular capsules, ii) supramolecular polymers, iii) supramolecular gels, iv) artificial host molecules, and v) catalytic reactions. Keywords: host-guest systems; organocatalysts; self-assembly; supramolecular chemistry; ureas EN host-guest systems organocatalysts self-assembly supramolecular chemistry ureas 1 1 1 04/09/21 20210326 NES 210326 B The ideal hydrogen bonding ability b of the ureido group leads to unique functions. [Extracted from the article]

Adsorption experiments of organic dyes were performed using supramolecular hydrogels of an amphiphilic tris ‐ urea derivative. Cationic organic dyes were adsorbed efficiently; indeed, the adsorption of methylene blue was as high as 4.19   mol equivalents relative to the gelator. Two luminescence peaks were observed in the rhodamine 6G ‐ adsorbed supramolecular hydrogels, and their ratios varied with the amount of dye adsorbed. The results suggest that the adsorption proceeded gradually, that is, occurring initially on the fibers and later in the aqueous spaces of the supramolecular hydrogels. More information can be found in the Communication by Miki Hasegawa, Masamichi Yamanaka et   al. on page   2029 in Issue 16, 2017 (DOI: 10.1002/asia.201700708). [ABSTRACT FROM AUTHOR]

Dynamic covalent chemistry is an effective technique for the preparation of complex organic compounds. We succeeded in synthesizing a cage-shaped compound through the aggregation of two types of functional molecules. More specifically, a tris-imine 5 was quantitatively obtained by reacting a C 3 -symmetric trialdehyde 1 with a triamine 4 in acetonitrile in the presence of a trifluoroacetic acid catalyst. We also achieved the synthesis of the corresponding triamine 9 and the acetylated derivative 10 through reduction of the tris-imine C N double bonds and subsequent acetylation. [ABSTRACT FROM AUTHOR]

Abstract: Tetra(4-pyridyl)-cavitand 1 and tetrakis(4-hydroxyphenyl)-cavitand 2 self-assemble into a heterodimeric capsule 1·2 via four PhOH⋯pyridyl hydrogen bonds in CDCl3, wherein one molecule of 1,4-disubstituted-benzene as a guest is encapsulated to form a ternary complex, guest@(1·2). The X-ray crystallographic analysis of (methyl p-ethoxybenzoate)@(1·2) confirmed that the methyl ester and ethoxy groups of the encapsulated guest are oriented to the cavity ends of the 1 and 2 units, respectively. The scope and limitation of guest encapsulation in 1·2, including guest-binding selectivity and orientational isomeric selectivity, are described from the viewpoint of size complementarity and CH–π, CH–halogen, and halogen–π interactions between guest and the cavity of 1·2. [Copyright &y& Elsevier]